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Red light, green light

Hors­es see a dif­fer­ent world than we do, in large part because of spe­cif­ic dif­fer­ences in their eyes. This arti­cle will focus on one aspect, the way horse and human eyes react to the col­ors of illu­mi­na­tion used for enhanc­ing human night vision. If you’re not inter­est­ed in the details, you can skip straight to the rec­om­men­da­tions.

Like humans, hors­es have two types of light‐​sensitive cells in their reti­nas (the reti­na is the light‐​sensitive lay­er at the back of the eye). Rods are sen­si­tive to very low lev­els of light, but don’t dis­crim­i­nate col­ors. Cones do the col­or dis­crim­i­na­tion, but need more light than rods to do their job. In humans, rods are about a hun­dred times more light‐​sensitive than cones, and there’s no rea­son to expect hors­es to be any dif­fer­ent. Vision involv­ing the cones is called pho­topic vision and vision involv­ing the rods is sco­topic vision.

Photopic vision (colors)

Humans have three dif­fer­ent types of cone cells that absorb light of three dif­fer­ent wave­lengths: 420 nm (nanome­ters), which appears blue, 534 nm, which appears green, and 564 nm, which appears red. These are “peak sen­si­tiv­i­ties”; each type of cone detects wave­lengths short­er and longer than its peak, and there’s some over­lap. We have “trichro­mat­ic vision”, and all the dif­fer­ent col­ors we per­ceive are the result of our brains pro­cess­ing the sig­nals from these blue, green, and red cones.

Humans and oth­er pri­mates are unique among mam­mals in hav­ing trichro­mat­ic vision. Hors­es, dogs, and oth­er mam­mals have only two kinds of cone cells, and dichro­mat­ic vision. In hors­es, blue‐​sensitive cones are most absorp­tive at 428 nm, and the oth­er type of cone is sen­si­tive at 539 nm, which appears yellow‐​green to us. Some humans have protanopia, a type of color‐​blindness in which the red cones don’t func­tion; a horse sees the world much like that.

Impor­tant for our pur­pos­es here, the yellow‐​green cone can’t detect light with a longer wave­length than around 640 nm, which is a deep red.

Scotopic vision (“night vision”)

Hors­es have excep­tion­al night vision rel­a­tive to humans. Their eyes are larg­er than most oth­er land mam­mals, to gath­er more light. They have a tape­tum lay­er that reflects light back onto the reti­na (this is what caus­es eye­shine). And their reti­nas are rich­er in rods. In envi­ron­ments with moon­light, starlight, or even reflect­ed city lights, hors­es can see well enough to walk, trot, or even gal­lop with­out run­ning into things.

When a rod absorbs light, its visu­al pig­ment (rhodopsin, or “visu­al pur­ple” in old­er ref­er­ences) is irre­versibly changed (“bleached”), so that the cell’s abil­i­ty to detect light is reduced until new rhodopsin is syn­the­sized. In low light, rhodopsin is regen­er­at­ed as fast as it is used, but a burst of bright light can bleach all the rhodopsin in all the rod cells, and it can take thir­ty min­utes or more for the cells to ful­ly regain their abil­i­ty to func­tion. For rea­sons that are not entire­ly clear to me, humans regain a use­ful lev­el of night vision with­in five min­utes of being exposed to bright light, but hors­es need the full thir­ty min­utes. The same horse that can run across a pas­ture under the quar­ter moon can­not eas­i­ly dis­crim­i­nate details in a dark horse trail­er when asked to enter it from bright day­light.

Illumination for night vision

Humans have poor­er night vision than many oth­er ani­mals, and there are sit­u­a­tions where it’s advan­ta­geous to main­tain night vision even in the pres­ence of illu­mi­na­tion. There are two dif­fer­ent approach­es in cur­rent use.

The first to be devel­oped was the use of red light. Rod cells are insen­si­tive to red light, so they can stay adapt­ed to low‐​light con­di­tions even while the red cone cells form images. Red night­time illu­mi­na­tion is still used in avi­a­tion and some mil­i­tary set­tings, and in dis­plays of some elec­tron­ic devices.

Red light has a cou­ple of dis­ad­van­tages for this use: it requires a high­er bright­ness, since it relies on pho­topic vision, and it’s some­times dif­fi­cult to dis­crim­i­nate shapes and near­ly impos­si­ble to dis­crim­i­nate col­ors.

A new­er alter­na­tive is to use bluish‐​green light, at about the peak sen­si­tiv­i­ty of rod cells. Because this is the light that rods see best, extreme­ly low inten­si­ties can improve sco­topic vision with­out destroy­ing dark adap­ta­tion. In a sense, it’s like car­ry­ing around a full moon.

The effect of these two types of light on hors­es is total­ly dif­fer­ent. Like humans, hors­es can­not see red with their rod cells, but they also can­not see it (or see it only dim­ly if it’s more of an orange‐​red) with their yellow‐​green cone cells. A human anal­o­gy is an infrared remote for an enter­tain­ment sys­tem: most peo­ple can­not see the beam even if it is shin­ing direct­ly in their eyes. For hors­es, deep red is “infra‐​yellow‐​green”, not real­ly a per­ceived col­or at all.

Red light pre­serves hors­es’ night vision. At a bright­ness lev­el high enough to be use­ful to humans, green light impairs their night vision.

And because hors­es have more rods, and more sen­si­tive rods, than humans, a low‐​intensity bluish‐​green light, bare­ly per­cep­ti­ble to a human, would be much brighter to a horse, and would reduce their night vision.

Recommendations

Red light pre­serves both human and horse night vision. Hors­es only see it dim­ly if at all, and con­tin­ue to rely on their superb night vision. It allows humans to per­ceive shapes and see text and dia­grams that would be much less vis­i­ble with the infe­ri­or human night vision, but at the same time pre­serves that night vision for see­ing in areas where the beam is not direct­ed.

The bluish‐​green light used for enhanc­ing human night vision is inap­pro­pri­ate for hors­es. It is only effec­tive for pre­serv­ing night vision in humans at the low­est lev­el of bright­ness that a human can use­ful­ly per­ceive, and this is much brighter that the low­est lev­el that a horse can per­ceive. If the human can see it, the horse’s night vision is already impaired.

Red head­lamps or flash­lights are use­ful for night trail rid­ing; they have no effect on the horse’s night vision, and aug­ment the human’s night vision. The use of white light head­lamps or flash­lights is strong­ly dis­cour­aged; they destroy night vision in both hors­es and humans in exchange for pro­vid­ing only a nar­row area of illu­mi­na­tion, and when they are switched off, hors­es need much longer to regain night vision than humans, and are effec­tive­ly stum­bling in the dark until they do.

Red LED illu­mi­na­tion may be more use­ful for barns and out­door sta­bles than the usu­al flu­o­res­cent light­ing. It allows a human to deal with a spe­cif­ic horse with­out destroy­ing the night vision of all the oth­er hors­es in the facil­i­ty. Hors­es dif­fer in their response to “hav­ing the lights turned on”, but even for the most non­re­ac­tive horse, it’s one more thing to put up with. In my ide­al facil­i­ty, there would be red light illu­mi­na­tion every­where that would go on with a sin­gle switch, so I could walk down the aisle with­out trip­ping on some­thing. Each stall could have an indi­vid­ual white flood lamp, or I could car­ry a white head­lamp or flash­light, if I need­ed bet­ter vision for a spe­cif­ic horse.

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2 thoughts on “Red light, green light”

This was very inter­est­ing. Now ques­tions:
For rid­ing at night (tail miles or in unlit school­ing area) what do I use for a head lamp?
For trai­ler­ing at night, I nor­mal­ly leave a incan­des­cent light on in the horse area to off­set the vehi­cle lights. What col­or light is best for the horse area in an open stock trail­er that will be ok for the hors­es but will not dis­tract dri­vers and increase the chance of acci­dents?

In answer to your first ques­tion, a lot of LED head­lamps include a red set­ting (mine cycles through red, bright white, very bright white, and annoy­ing­ly blink­ing white). But the red set­ting is often not very bright, and is only use­ful at close range. I bought an extreme­ly bright red LED flash­light that works well, and I’m hop­ing to some­day track down a red head­lamp that bright.

In answer to your sec­ond ques­tion, I was just talk­ing to a friend about that yes­ter­day; I had nev­er real­ly thought about the issue of vehi­cle lights in a trail­er. The prob­lem is that vehi­cle lights will mess up their night vision, leav­ing them in the dark for the rest of the time, and there’s no way to con­trol that. Since no one makes red gog­gles or trans­par­ent red fly masks for hors­es 😀, I think the best approach is to use a bright enough light to give them pho­topic vision dur­ing the times when they’re not illu­mi­nat­ed by head­lights. As far as not dis­tract­ing oth­er dri­vers, it occurs to me that amber might be a good col­or, since the side run­ning lights are often amber.

If the hors­es are unloaded into a light­ed area (which is pret­ty ordi­nary) and stalled for the rest of the night, they should have no prob­lems. But if they need to work in the dark after they come off the trail­er, they’ll need 30 min­utes to adapt.